1. Field of the Invention
The present invention relates to a method of fabricating an optical fiber preform using a modified chemical vapor deposition method and a nonlinear optical fiber fabricated using the method, in which the optical fiber is provided with a specific function by adding a particular dopant.
2. Description of the Related Art
An optical fiber utilized in optical communications is an element, in which light is transmitted by means of total reflection due to a difference of optical refractive indices between a cladding portion, which is made from quartz glass of high purity, and a core portion, in which elements such as germanium (Ge) are added to silica glass to slightly heighten the optical refractive index.
In general, the process of fabricating the optical fiber is further divided into a process of fabricating an optical fiber preform and a process of drawing an optical fiber from the optical fiber preform. The process of fabricating an optical fiber preform is accomplished by utilizing such methods as a modified chemical vapor deposition (MCVD) method, a vapor-phase axial deposition (VAD) method, and an outside vapor deposition (OVD) method.
Further, in order to provide the optical fiber with a particular functionality, after a core layer is deposited and partially-sintered in an MCVD process, a solution containing a dopant is added to the sintered portion of the core layer.
FIG. 1 is a flow chart for showing a process of fabricating a functional optical fiber preform according a general MCVD method.
First, a raw gas such as SiCl4, POCl3, CF4, and GeCl4 together with oxygen is blown into a quartz glass tube, and the quartz glass tube is heated by a heating means. Then, oxidized sediment like soot is formed on the inside of the quartz glass tube due to a thermal oxidation reaction, thus forming a cladding layer and a core layer (steps ST1 and ST2).
Thereafter, the core layer is partially sintered, and then doped with a dopant, so as to have a particular functionality (step ST3).
Further, the portion doped with the dopant is dried, and sintered accompanying oxidation (step ST4).
Thereafter, the sintered optical fiber is subjected to a collapsing step and a sealing step, so that the fabrication of an optical fiber preform is completed (steps ST5 and ST6).
In this case, the above doping steps ST3 and ST4 are carried out by means of an apparatus as shown in FIG. 2, described in detail hereinafter with reference to FIGS. 3A to 3D.
As shown in FIG. 3A, a cladding layer 32 and a core layer 33 are formed on the inside quartz glass tube 31, and as shown in FIG. 2, the quartz glass tube 31 is connected to a flask 10 through a Teflon connector 20.
In this case, the flask 10 contains a solution S containing a dopant to dope the quartz glass tube 31, and has a gas injection/exhaust port 11, through which a suitable gas such as argon (Ar) is injected to supply the solution S into the quartz glass tube 31.
In other words, in a state that the quartz glass tube 31 and the flask 10 are connected with each other through the Teflon connector 20, when a predetermined quantity of argon gas is injected into the gas injection/exhaust port 11 of the flask 10, the solution S contained in the flask 10 is injected into the quartz glass tube 31 through the Teflon connector 20 by the pressure due to the injection of the gas. That is, the quartz glass tube 31 is maintained in a state as shown in FIG. 3B.
Thereafter, when a predetermined time has passed, the argon gas is exhausted through the gas injection/exhaust port 11 of the flask 10. Then, the solution S remaining in the quartz glass tube 31 returns to the flask 10 through the Teflon connector 20. That is, as shown FIG. 3C, the sintered portion of the core layer is doped with the solution.
However, since the apparatus shown in FIG. 2 is to carry out only the doping process, other processing steps such as the steps of forming a cladding layer and a core layer in a quartz glass tube, a sintering step, a collapsing step, and a sealing step must be carried out by a conventional apparatus for the MCVD process.
Therefore, after a cladding layer and a core layer are formed in a quartz glass tube by means of a conventional MCVD processing apparatus, the quartz glass tube is removed to an additional apparatus to carry out the doping process. Then again, the quartz glass tube having completed the doping step is installed in the conventional MCVD processing apparatus for the subsequent steps, thus complicating the process of fabricating an optical fiber perform.
Moreover, when the apparatus, as shown in FIG. 2, carries out the doping process, the quartz glass tube is set upright, filled with the solution containing a dopant for a predetermined time, and then exhausted, so that some of the dopant filling the sintered portion of the core tends to escape along with the solution. This results in deterioration of the functional characteristic imparted by the doping of the optical fiber perform, as shown in FIG. 3D.
Meanwhile, the recent development of the optical communications technology requires high-speed nonlinear optical elements such as an optical modulator, an optical switch, and an optical isolator. As an endeavor in order to produce such nonlinear optical elements, a research for doping an optical fiber with semiconductor fine particles is in progress.
However, in the conventional method of fabricating an optical fiber preform as described above, there is a difficulty in doping semiconductor fine particles of nano-size, which is larger than that of existing dopants.
Further, the doping apparatus as described above limits the sintered portion of the core layer, and thus limits doping quantity of the dopant.
This adversely affects the characteristic of a functional optical fiber, especially that of the non-linear optical fiber, which is influenced by the doping quantity of the dopant.